1. Field of the Invention
The present invention relates to a process for preparing 4-aminodiphenylamines. It also relates to a process for preparing alkylated derivatives of said 4-aminodiphenylamines.
2. Related Art
4-Aminodiphenylamines are widely used as intermediates in the manufacture of alkylated derivatives having utility as antiozonants and antioxidants, as stabilizers for monomers and polymers, and in various specialty applications. For example, reductive alkylation of 4-aminodiphenylamine (4-ADPA) with methylisobutyl ketone provides N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylene-diamine, which is a useful antiozonant for the protection of various rubber products.
4-Aminodiphenylamines can be prepared in various ways. An attractive synthesis is the reaction of an optionally substituted aniline with an optionally substituted nitrobenzene in the presence of a base followed by catalytic hydrogenation of the reaction product, as disclosed, for example, in U.S. Pat. No. 5,608,111 (to Stern et al.) and U.S. Pat. No. 5,739,403 (to Reinartz et al.).
U.S. Pat. No. 5,608,111 describes a process for the preparation of an optionally substituted 4-ADPA wherein optionally substituted aniline and optionally substituted nitrobenzene are reacted (coupled) in the presence of a base. Subsequently, water is added to the coupling product and the resulting reaction mixture is catalytically hydrogenated. The catalyst, which typically is a supported noble metal catalyst, is removed from the hydrogenation reaction mixture, the organic phase is separated in order to isolate the 4-ADPA, and the aqueous phase, which contains the base, is returned to another cycle of the initial reaction mixture. In working examples, aniline and nitrobenzene are reacted in the presence of tetramethylammonium hydroxide as the base, and water and aniline are azeotropically removed during the coupling reaction.
U.S. Pat. No. 5,739,403 describes a process for a coupling reaction comparable to Example 13 of U.S. Pat. No. 5,608,111 and subsequent catalytic hydrogenation, where the amount of water added is 25 to 80 wt. % relative to the weight of the coupling (condensation) reaction mixture. Example 1 of U.S. Pat. No. 5,739,403 is distinguished from Example 13 of U.S. Pat. No. 5,608,111 in that it teaches a 4-hour hold period, with continued distillation, after completion of nitrobenzene addition. After the hydrogenation, toluene is added, the catalyst is filtered out, and the organic and aqueous phases are separated. In Example 1 it is stated that, "Analysis of the aqueous phase shows that 99.7% of the introduced tetramethylammonium hydroxide may be isolated. The resultant aqueous phase may be returned to the reaction without loss of reactivity."
One of the objectives of the present invention is that of base recovery and of recycling the base. U.S. Pat. No. 5,739,403 provides no teaching with regard to recycling the base with as little loss of reactivity as possible (including effect on hydrogenation catalyst activity). Since the document does not disclose a second cycle of coupling and hydrogenation reactions, the reactivity of the base recovered in the process disclosed is in fact unknown.
A further disadvantage of the process disclosed in U.S. Pat. No. 5,739,403 is that a relatively large amount of an aromatic solvent is used for separating the organic and aqueous phases. The use of large amounts of an organic solvent in unit operations like separation of layers is highly undesirable in commercial scale processes because of the costs involved in recovering and processing such solvent. Therefore, another objective of the present invention is to facilitate separation of the organic and aqueous phases.
Another objective of the present invention is that of recycling the hydrogenation catalyst and the excess aniline. None of the references cited above touches upon this feature.
A further objective of the present invention is that of handling the formation of any by-products, such as azobenzene and azoxybenzene, which may or may not occur.